A rubber vulcanizate (molded or formed product of conductive rubber) obtained by forming a rubber composition, in which conductive particles have been incorporated, into a predetermined shape and then vulcanizing it is required to control the hardness and electric resistance value thereof within respective predetermined ranges according to the use application thereof. However, the rubber vulcanizate containing the conductive particles has involved a problem that when a compounding proportion of the conductive particles is made high to achieve a sufficiently low electric resistance value, its hardness becomes too high. When the hardness thereof is controlled by adding a compounding ingredient such as a softener in a great amount, there is offered a problem that the compounding ingredient bleeds out into the surface of the molded or formed product of the conductive rubber. Accordingly, a difficult problem is encountered on the provision of a molded or formed product of conductive rubber, which combines good electrical conductivity with good softness.
When the desired properties cannot be achieve by only one rubber component, it is generally conducted to use a plurality of rubber materials in combination. Even when at least two rubber components are used in combination, however, it is difficult to obtain a molded or formed product of conductive rubber combining good electrical conductivity with good softness by an ordinary method in which vulcanizing agents corresponding to the respective rubber components are used to vulcanize the rubber components at the same time. More specifically, when the individual rubber components are compatible with each other, the hardness of the resulting molded or formed product becomes high because conductive particles are evenly dispersed in the rubber components. Even when the individual rubber components are incompatible with each other, the hardness of the resulting molded or formed product becomes high because the conductive particles are also evenly dispersed in the rubber components unless there is a particular difference in affinity for the conductive particles between the rubber components.
When the individual rubber components are incompatible with each other, and there is a difference in affinity for the conductive particles between the rubber components, the conductive particles come to exist at a higher concentration in the phase of the rubber component having higher affinity for the conductive particles. In this case, the phase of the rubber component having lower affinity for the conductive particles and containing the conductive particles at a lower concentration contributes to good softness. However, the kinds of usable rubber components are naturally limited in the method of controlling the electrical conductivity and softness by the difference in the affinity for the conductive particles, so that it is difficult to satisfy other properties such as abrasion resistance.
Conductive members such as conductive rolls and conductive blades used in image forming apparatus such as electrophotographic copying machines and electrostatic recording apparatus are required to combine good softness with good electrical conductivity. Therefore, the above-described problems will hereinafter be described in more detail taking the case of these conductive members.
Image forming apparatus such as electrophotographic copying machines and electrostatic recording apparatus are used in various kinds of image forming methods such as electrophotography, electrostatic photography and electrostatic printing. In these image forming apparatus, conductive rolls are used as, for example, a development roll, a charging roll, a transfer roll and the like. Besides, conductive blades are used as a blade (doctor blade) for forming a toner layer, a cleaning blade and the like.
Properties required of various rolls used in an image forming apparatus vary according to the kind of the image forming apparatus, use applications of the rolls, and the like. However, properties commonly required thereof generally include first to have moderate softness, second to exhibit an electric resistance value stable to changes in environment, third not to contaminate a photosensitive member and the like, and forth to have excellent durability.
These rolls are required to have, as an index to softness, a hardness ranging from 20 to 60.degree. in terms of JIS-A hardness. If the hardness of these rolls is too high, such rolls tend to, for example, cause fusion bonding of a toner used in the image forming apparatus and damage a photosensitive member. If the hardness is too low, such rolls tend to, for example, fail to apply sufficient frictional force to the toner, and contaminate the photosensitive member. At the same time, these rolls are required to have an electric resistance value ranging from 1.times.10.sup.4 to 1.times.10.sup.10 .OMEGA. as an electrical property. For example, a development roll plays a role that a toner is caused to adhere to the peripheral surface of the development roll in a charged state by frictional force generated between the development roll and a developer feed roll, and the toner is evenly smoothed by a layer-forming blade and caused to fly toward an electrostatic latent image on the photosensitive member by electric attraction force. Therefore, the development roll must have an electric resistance value within a proper range. In the case of a charging roll, it fails to evenly charge the surface of the photosensitive member unless it has an electric resistance value within a proper range. In addition, such rolls are required to make less difference in electric resistance value between under high temperature and high humidity conditions of, for example, 35.degree. C. and 80% RH (relative humidity) and under low temperature and low humidity conditions of, for example, 10.degree. C. and 20% RH from the requirement for formation of a high-quality copy image, i.e., to have excellent environmental resistance and exhibit a stable electric resistance value even by changes in surrounding environmental conditions.
Therefore, in various image forming apparatus, conductive rubber materials which combine good softness with good electrical conductivity are used as materials for conductive rolls. As the conductive rubber materials, there are used rubber compositions obtained by incorporating conductive particles such as conductive carbon black into various kinds of rubber. To these conductive rubber materials, a softener and the like may be further added in some cases with a view toward satisfying the standard of softness, i.e., 20-60.degree. in terms of JIS-A hardness.
As conductive rubber materials used in the above-described use applications, there have heretofore been proposed rubber compositions in which conductive particles are separately incorporated into, for example, silicone rubber (Japanese Patent Application Laid-Open No. 200158/1994), isoprene rubber (Japanese Patent Application Laid-Open No. 224501/1993), ethylene-propylene rubber (Japanese Patent Application Laid-Open No. 188753/1993) and polyhexene rubber (Japanese Patent Application Laid-Open No. 133937/1992). However, sufficient properties have not been yet achieved even by these conductive rubber materials. Besides, a conductive rubber material (Japanese Patent Application Laid-Open No. 190964/1991) in which two kinds of silicone rubber are used in combination as a rubber component, a contact-charging member (Japanese Patent Application Laid-Open No. 196067/1991) equipped with a conductive layer containing polyolefin rubber and silicone rubber, and the like have been proposed. However, these conductive rubber materials have drawbacks of insufficient abrasion resistance and poor durability.
By the way, conductive carbon black is most commonly used as the conductive particles. However, when the conductive carbon black is incorporated into conjugated diene rubber in a quantitative proportion sufficient to achieve the desired electrical conductivity, the hardness of the resulting vulcanized rubber may be increased to higher than 60.degree. in terms of JIS-A hardness in some cases due to interaction between the conductive carbon black and the conjugated diene rubber. When a softener, flexibilizer or the like commonly used in the field of rubber materials is incorporated into a conductive rubber material, the hardness of the resulting rubber material can be controlled within a proper range. However, the compounding ingredient such as the softener bleeds out, so that problems of contamination of a photosensitive member, and the like arise.
It has been proposed to incorporate liquid rubber in a great amount in place of the softener or flexibilizer to control the hardness of the resulting conductive rubber material (Japanese Patent Application Laid-Open No. 59222/1993). In order to achieve sufficient softening action, however, it is necessary to incorporate the liquid rubber in a comparatively great amount into the conjugated diene rubber. Therefore, the molding and processing ability of the resulting rubber composition is deteriorated, and moreover a sticky feel is left on the surface of a molded product from the rubber composition due to low molecular weight substances in the liquid rubber.
In order to prevent the bleedout of compounding ingredients such as a softener, it has been proposed to, for example, provide a bleedout-preventing layer (Japanese Patent Application Laid-Open No. 250496/1994), a protective layer (Japanese Patent Application Laid-Open No. 208288/1994), or a surface layer (Japanese Patent Application Laid-Open No. 167861/1994), or laminate a resistance-regulating layer (Japanese Patent Application Laid-Open No. 208289/1994) on a rubber layer having good electrical conductivity. These methods are effective methods for preventing the bleedout, but unpreferable methods from the viewpoint of practical use because the number of steps is increased.
Accordingly, there is a demand for development of a conductive rubber material which has an electric resistance value within the desired range by containing a predetermined amount of conductive particles, permits controlling even its hardness within the above-described standard and can be used in the form of a single layer. Conductive blades are also required to have the same properties as in the conductive rolls. However, neither any conductive rubber material which sufficiently satisfies these requirements, nor any satisfactory conductive roll has existed.